<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
            width: 500px;
            height: 400px;
            border: 1px solid;
            display: flex;
            /* 水平居中 */
            /* justify-content: center; */
            /* 垂直居中 */
            /* align-items: center; */
        }
        .container>div{
            width: 100px;
            height: 100px;
        }
        .container>div:nth-child(1){
            background-color: red;
        }
        .container>div:nth-child(2){
            background-color: yellow;
        }
        .container>div:nth-child(3){
            background-color: skyblue;
        }
    </style>
</head>

<body>
    <div class="container">
        <div></div>
        <div></div>
        <div></div>
    </div>
</body>

</html>